In oil well evaluation and aquifer management, quantitative analyses of formation fluid are typically performed in a laboratory environment, the samples having been collected remotely. Standard laboratory procedures are available for quantitative analyses by adding a reagent to chemically react with a specific target species in a sample to cause detectible changes in fluid property such as color, absorption spectra, turbidity, electrical conductivity, etc. See Vogel, A. I., “Text-Book of Quantitative Inorganic Analysis, 3rd Edition”, Chapter 10-12, John Wiley, 1961, incorporated by reference herein in its entirety. Such changes in fluid property may be caused, for example, by the formation of a product that absorbs light at a certain wavelength, or by the formation of an insoluble product that causes turbidity, or bubbles out as gas. For example, addition of pH sensitive dyes is used for colorimetric pH determination of water samples. A standard procedure for barium determination requires addition of sodium sulfate reagent to the fluid sample resulting in a sulfate precipitate that can be detected through turbidity measurements. Some of these standard laboratory procedures have been adapted for flow injection analysis (Ruzicka et al., Flow Injection Analysis, Chapters 1 and 2, John Wiley, 1981, incorporated by reference herein in its entirety). Flow injection analysis “is based on the injection of a liquid sample into a moving non-segmented continuous carrier stream of a suitable liquid” (see Ruzicka et al., Chapter 2, page 6).
Fluid samples collected downhole can undergo various reversible and irreversible phase transitions between the point of collection and the point of analysis as pressure and temperature conditions are hard to preserve. Concentrations of constituent species may change because of loss due to vaporization, precipitation etc., and hence the analysis as done in the laboratories may not be representative of true conditions. For example, water chemistry and pH are important for estimating scaling tendencies and corrosion; however, the pH can change substantially as the fluid flows to the surface. Likewise, scaling out of salts and loss of carbon dioxide and hydrogen sulfide can give misleading pH values when laboratory measurements are made on downhole-collected samples. Conventional methods and apparatuses require bulky components that are not efficiently miniaturized for downhole applications.
Further, fluid sample for water management requires very frequent (i.e. daily, twice daily, etc.) monitoring and measuring of fluid properties. These monitoring regimes include permanent subsurface systems that are designed solely to gather and store frequently acquired data over long periods of time. Accordingly, there is a need for a system that uses very low quantities of reagent, operates autonomously, and collects or neutralizes waste product. Traditional solutions include chemical sensors that tend to lose calibration over a relatively short period of time.
As will be described in more detail below, the present invention applies MEMS/MOEMS techniques to develop microfluidic devices overcoming the limitations of the prior art. Micro electromechanical systems (MEMS) are well known as microfluidic devices for chemical applications since the 1990's (see Manz et al., “Miniaturized Total Chemical and Analysis Systems: A Novel Concept for Chemical Sensing,” Sensors and Actuators B, Vol. B1, pages 244-248 (1990), incorporated by reference herein in its entirety) and are typically fabricated from silicon, glass, quartz and poly(dimethylsiloxane) (PDMS) (see Verpoorte et al., “Microfluidics Meets MEMS” Proceedings of the IEEE, Vol. 91, pages 930-953 (June 2003), incorporated by reference herein in its entirety). MEMS technology allows for miniaturized designs requiring smaller liquid volumes. In addition, MEMS devices are easy to mass produce having a very accurate reproducibility. MEMS also allows easy integration of different components, such as valves, mixers, channels, etc. Similarly, MEMS systems with optical devices are called MOEMS (micro optical electro mechanical systems, or Optical MEMS). MOEMS have also been used for chemical applications since the 1990's. Commercial (non-chemical) structures are used in the telecommunications field to make use of MEMS wave-guides to modify or route an optical signal.
For example, U.S. Pat. No. 5,116,759 to Klainer et al. (incorporated by reference herein in its entirety) discloses a laboratory-based system utilizing a MEMS device. In particular, the MEMS device is a cell that receives the sample for analysis. All associated analytical devices, including optical interrogation, power supply, reagent sources, and processing means, are typical laboratory-sized devices not suitable for remote interrogation.
Accordingly, it is one object of the present invention to provide a novel system to autonomously perform remote chemical analysis.
It is another object of the present invention to provide a microsystems that will regulate the amounts of sample and reagent to be consumed during each measurement, allowing the use of a reagent reservoir in the downhole instrument and the storage of waste within the instrument.
It is yet another object of the present invention to provide a microsystem having a total flow rate in the order of microliters per minute, enabling the measurement of pH and use with other reagents for determining the concentration of species like nitrate, heavy metals, scaling ions and hydrocarbons.
It is yet a further object of the present invention to provide an autonomous system having low power consumption, minimum consumables, neutralized waste material and data logging for in-situ measurements of fluid parameters on a multi-year permanent basis.